1. Technical Field
Embodiments of the invention relate to a metal separator for fuel cells and, more particularly, to a metal separator for fuel cells and a method of manufacturing the same, in which a metal element and a carbon element are coated on a separator-shaped metal matrix such that a gradient is formed whereby the carbon element is concentrated on the surface of the metal matrix, or in which a buffer layer is formed using metal on the separator-shaped metal matrix and an amorphous carbon layer is coated on the buffer layer, thereby enhancing properties of the metal separator such as corrosion resistance, electrical conductivity, durability, and the like.
2. Description of the Related Art
A fuel cell is an electrochemical cell that converts chemical energy produced by oxidation of fuel into electrical energy. Recently, various investigations have focused on development of fuel cells, solar cells, and the like in order to overcome problems such as consumption of fossil fuels, the greenhouse effect and global warming caused by carbon dioxide, and the like.
Fuel cells generally convert chemical energy into electrical energy through oxidation and reduction of hydrogen and oxygen. In the fuel cell, hydrogen is oxidized into hydrogen ions and electrons at an anode, and the hydrogen ions diffuse to a cathode through an electrolyte. The electrons travel to the cathode through a circuit. At the cathode, water is produced through reduction of the hydrogen ions, electrons, and oxygen.
In general, since a unit cell of a fuel cell generates too low a voltage to be used alone in practice, the fuel cell has several to several hundred unit cells stacked therein. When stacking the unit cells, a bipolar plate or separator is used to facilitate electrical connection between unit cells and to separate reaction gases.
The separator for fuel cells may be classified into a graphite separator, a metal separator, and the like according to the material of which the separator is made.
The graphite separator has been widely employed for a conventional separator for fuel cells. The graphite separator is manufactured by milling graphite according to the shape of a flow passage. In this case, the graphite separators in the stack represent 50% of the total manufacturing costs and 80% of the total weight of the stack. Accordingly, the graphite separator has problems of high manufacturing costs and large volume.
To solve such problems of the graphite separator, metal separators have been developed. Metal separators have many advantages, such as easy processibility, overall reduction in volume and weight of a fuel cell stack through thickness reduction of the separator, mass productivity, and the like.
In this case, however, the metallic material of the separator inevitably undergoes corrosion during use of the fuel cell, causing contamination of a membrane electrode assembly and performance deterioration of the fuel cell stack, and a thick oxide film can grow on the surface of the metal separator after extended use of the fuel cell, causing an increase in internal resistance of the fuel cell.
Therefore, there is a need for a metal separator for fuel cells, which has high corrosion resistance and electrical conductivity to improve performance of the metal separator by suppressing corrosion of the metal and the increase in internal resistance of the fuel cell.